Our universe

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#1
Despite James webb and other very impressive advances in technology the ultimate fate of our universe remain up in the air.

Currently the mainstream ΛCDM model suffer from severe problems with no clear replacement in sight.

Will our universe end via heath death, big rip or big crunch? Will it perhaps be something else we are not even considering now? nobody knows fur sure.
 

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#2
The Boltzmann brain problem
One perhaps controversial tool to rule out some universes is universes where boltzmann brains over time dominate when it comes to consciouss enteties. This should rule out the scenario of eternal exponential expansion.

https://arxiv.org/pdf/0802.0233.pdf

 

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#3
The Hubble tension
The value of H0 predicted from the cosmological background radiation very much does not agree with what we are observing today. This 'problem' has persisted for years and has only gotten more significant over time.

https://arxiv.org/pdf/2302.05709.pdf

https://arxiv.org/abs/1907.10625

https://bigthink.com/starts-with-a-bang/jwst-confirms-worsens-hubble-tension/

https://arxiv.org/pdf/1910.02978.pdf





https://arxiv.org/abs/2311.00215

Note that while there have been attempts at resolving this via new physics many of those approaches simply doesn't work, at least not alone.

https://arxiv.org/abs/2003.07355
 

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#4
The big rip model
If dark energy grows stronger over time that might at least in part explain the Hubble tension but there are also problems.

For one it's unclear if there is even any particularly great underlying theoretical explanation for phantom dark energy.

https://arxiv.org/abs/1708.06981


I have seen conflicting statements regarding if phantom dark energy would actually resolve the Hubble tension alone or if w is too constrained resulting in other solutions still being required even if w ≠ -1.

https://arxiv.org/pdf/2005.12587.pdf

https://diposit.ub.edu/dspace/bitstream/2445/176715/1/NOVELL MASOT SERGI_474060.pdf

https://arxiv.org/abs/2004.08363

https://www.mdpi.com/1099-4300/23/4/404

https://arxiv.org/pdf/2205.13514.pdf

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8947162/

https://arxiv.org/pdf/2306.08046.pdf

Examples of papers stating that w < -1 wouldn't work particularly well:

Given that the scaling of the radiation and matter densities with redshift is known, this requires some exotic matter whose energy density increases with time. This is most easily accomplished by postulating that the cosmological constant is a phantom field (88), a fluid with an equation-of-state parameter w = p/ρ < −1, where p and ρ are the dark energy pressure and energy density. This, however, implies a fluid that violates the strong energy condition; that is, it effectively creates energy out of nowhere. This seems strange, but is this what the Hubble tension is telling us? Even if we are willing to accept a violation of the strong energy condition, such models are difficult to reconcile with the sound horizon seen in the galaxy correlation function (89, 90). They are also difficult to reconcile with constraints to the equation-of-state parameter w inferred recently from SNe Ia at high redshifts (25).

https://www.annualreviews.org/doi/full/10.1146/annurev-nucl-111422-024107

hubble-tension wCDM vs lamda-CDM.png


https://arxiv.org/pdf/2301.04200.pdf
 

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#5
Big rip cyclic cosmology
A big rip could lead to a new big bang opening up the door for big rip cyclic cosmology. This could allow for laws of physics to change with each new universe resulting in the universe sometimes after a big rip having laws of physics that actually allow for life.

https://medium.com/starts-with-a-bang/ask-ethan-could-the-big-rip-lead-to-another-big-bang-566a4fbb0f25

It also seems like this cyclic model doesn't have a problem with entropy unlike other cyclic universe attempts.

https://arxiv.org/pdf/hep-th/0703162.pdf


https://arxiv.org/abs/2005.12684

https://www.researchgate.net/publication/236215355_Cyclic_Cosmology_from_the_Little_Rip
 

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#6
Apparent dark matter
When we look at the effects of gravity there seems to be a lot of matter there that we cannot detect in any other way.

The most likely explanation for this is that there are particles that are interacting extremely weakly (such as only via gravity) and this wouldn't exactly be a revolutionary idea since we already know about extremely weakly interacting neutrinos.

Interestingly in particle physics extremely weakly interacting axions has already been proposed to solve the strong CP problem and they seem to do a better job at explaining astronomical measurements than weakly interacting massive particles.


https://arxiv.org/abs/2203.14923


https://arxiv.org/abs/2311.16377
 

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#7
How should "planet" be defined?
The current official IAU definition of planet doesn't really make sense for multiple reasons.

0. Objects can be "exoplanet" or dwarf planet" without actually being a planet, that simply doesn't make any grammatical sense.

1. Whether or not an object is a planet depends on it's orbit/position.


The least problematic criteria is with regard to hydrostatic equilibrium but even then it would be better to go on mass.

The biggest problem with going only on mass is that it's often hard to measure that. With Sedna for example we can only guess the mass based on the density of pluto and it's size but we currently do not have anything close to an actual measurements of it's mass.

With exoplanets however we sometimes get a more accurate figure for its mass since we discover it by looking at how it gravitationally affects its nearest star.

One way to differentiate moons + planet systems from planet + planet systems is to use the following

it's a binary system if the barycenter of the objects' orbits is not within one of the objects. That's the line between binary system and planetary system. In that case, neither object is a moon and they co-dominate their orbit, so they'd both be a part of a binary planet system.
That is currently not followed strictly since Charon is often described as a moon of pluto even though the barycenter is outside Pluto. We need to follow that more strictly.

My suggestion is to use the following to determine what counts as major moons/planets and what counts as minor moons/planets.

Major moon/planet: Any object with radius of at least 240 Km in all directions from it's centre of mass, mass more than 2*10^22 and mass smaller than 2.4 * 10^28 Kg

Minor moon/planet: Any object with radius of at least 240 Km and mass less than 2 * 10^22 Kg

Satelite Asteroid: sufficiently large natural sattelite that

That would result in the following 8 major planets

Terrestrial planets
Mercury
Venus
🜨 Earth
Mars

Gas giants
Jupiter
Saturn
Uranus
Neptune

You would also get 7 major moons

The Moon orbiting Earth.
Io, Europa, Ganymede, and Callisto orbiting Jupiter.
Titan orbiting saturn.
Triton orbiting Neptune.

With the following moons being in the minor category regardless

Mimas, Enceladus, Tethys, Dione, Rhea, and Iapetus orbiting Saturn.
Miranda, Ariel, Umbriel, Titania, and Oberon orbiting Uranus.

1702942494668.png


https://en.wikipedia.org/wiki/List_of_Solar_System_objects_by_size

Minor planets aka dwarf planets
Ceres
Orcus
Pluto
Charon technically in a binary system with Pluto.
Haumea
Quaoar
Makemake
Gonggong
Eris
Sedna
Salacia

1702993470858.png


Note that you would basically get the same outcome by having a minimum diameter of 1200Km for something to be considered a "major moon/planet" since none of the moons/planets smaller diameter than that has mass above 20*10^21 Kg.
 

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#8
The mass gaps for planets/moons in our solar system
When looking at just masses we see a very big gap between Pluto and Mercury for planets (excluding moons, in 10^21 Kg):

Jupiter: 1 898 187 ± 88
Saturn: 568 317 ± 13
Neptune: 102 413 ±5
Uranus: 86 813 ± 4
Earth: 5 972.4 ± 0.3
Venus: 4 867.5 ± 0.2
Mars: 641.71 ± 0.03
Mercury: 330.11 ± 0.02

Eris: 16.6 ± 0.2
Pluto: 13.03 ± 0.03
Haumea: 4.01 ± 0.04
Makemake: ≈ 3.1
Charon: 1.586 ± 0.015
Quaoar: 1.20 ± 0.05
Orcus: 0.548 ± 0.010
Salacia: 0.492 ± 0.007

With moons we do see a big gap between Triton and Titania:

Ganymede: 148.2
Titan: 134.5
Callisto: 107.6
Io: 89.32
Moon: 73.46
Europa: 48.00
Triton: 21.39 ± 0.03

Titania: 3.40±0.06
Oberon: 3.08±0.09
Thea: 2.307
Iapetus: 1.806
Ariel: 1.25 ± 0.02
Dione: 1.095
Tethys: 0.617
Dysnomia: 0.3 to 0.5
Enceladus: 0.108 ± 0.1

https://en.wikipedia.org/wiki/List_of_Solar_System_objects_by_size

The figure 2*10²² Kg happen to be within both those mass gaps making it a natural criteria for "major moon/planet".

Having the same mass criteria for Major moon and major planet is beneficial since it allows us to write "major moon/planet" for any object with mass exceeding 20*10²¹ Kg.
 

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#9
Habitable planets
A big obstacle when it comes to trying to establish humanity on other planets is the difficulty in actually reaching any planet that is actually habitable.

Most planets orbit around red dwarf but red dwarf tends to release a lot of nasty flares to planets that are too close while planets further away do not receive enough light to be habitable in the first place.


Alpha Centauri A may have a candidate Neptune-sized planet in the habitable zone. If true we have to hope that said planet has some habitable moon (unlikely).
 

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#10
New study find W to be -0.8
This was very much not what we expected based on previous data.

https://noirlab.edu/public/news/noirlab2401/?lang


Earlier the skydivephil channel talked about how "Causal Set Theory" predicted variable dark energy where the universe would eventually end in a big crunch and now this is becoming very much relevant with these recent results.

https://youtu.be/3OxAb9ho3UE?si=9ZSGh-8z2X02wmL4&t=904

The "big crunch" was of course never actually ruled out but earlier there wasn't much of a reason to think it would actually take place.
 
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